1. Field of the Invention
This invention relates to a system for controlling electric power steering.
2. Description of Related Art
FIG. 6 shows a conventional system for controlling electric power steering. FIG. 7 is a block diagram showing the control in the system for controlling the electric power steering.
As illustrated in FIG. 6, a pinion 2 is placed on one end of an input shaft 1 associated with a steering wheel W. A rack is provided on a rod 4 associated with wheels 3R, 3L at two end portions. The pinion 2 on the input shaft 1 engages the rack 5 on the rod 4.
An electric motor S is associated with a reduction gear 7 having an output shaft on which a pinion is provided. The pinion also engages the rack 5 on the rod 4. A controller CT is connected to steering-torque detection means 8 for detecting steering torque acting on the input shaft 1.
As illustrated in FIG. 7, the controller CT includes basic assist command-value determination means 9 for determining a basic assist command-value in response to a steering torque signal from the above sensor 8, and steering-torque differentiation command-value determination means 10 for differentiating the steering torque detection signal.
The controller CT further includes output control means 11 for controlling the outputs of the electric motor 6. The output control means 11 includes motor-current amplification means 12 for amplifying an input signal, and digital-to-analog conversion means 13 for converting a digital signal into an analog signal. For example, the means 11 can be a pulse width modulation circuit made up of a combination of the motor-current amplification means 12 and the DA conversion means 13.
In order to detect the outputs of the electric motor 6, the output control means 11 has output wiring which is connected to motor current detection means 14. In order to return a motor-current detection signal from the motor-current detection means 14 to the controller CT, the controller CT incorporates analog-to-digital conversion means 15. In other words, the AD conversion means 15 converts the motor-current detection signal in analog form into a digital signal.
In this way, the motor-current detection signal is sent back to the controller CT under the feedback control.
With the above configuration, the control system operates for controlling the electric motor 6 as follows.
As illustrated in FIG. 7, the steering-torque detection signal from the steering-torque detection means 8 is inputted to the basic assist command-value determination means 9. Then, the basic assist command-value determination means 9 outputs a basic assist command-value signal in accordance with the received steering-torque detection signal. The steering-torque detection signal is also inputted to the steering-torque differentiation command-value determination means 10. Then, the steering-torque differentiation command-value determination means 10 outputs a steering-torque differentiation command-value signal in accordance with the received steering-torque detection signal. The steering-torque differentiation command-value signal is added to the above basic assist command-value signal to generate a motor current command-value signal.
The basic assist command-value signal and the steering-torque differentiation command-value signal are digital signals.
The motor current detection means 14 detects motor current outputted from the output control means 11, and sends the detected motor-current detection signal back to the input of the output control means 11 via the AD conversion means 15 placed in the controller.
The motor current detection signal is added to the above motor current command-value signal to generate a motor current control signal. The motor current control signal is inputted to the motor current amplification means 12 in the output control means 11. The motor current amplification means 12 amplifies the received motor current control signal, and then inputs the amplified motor current control signal to the DA conversion means 13.
The DA conversion means 13 outputs motor current according to the amplified motor current control signal to the electric motor 6.
In this way, the system for controlling the electric power steering controls the currents of the electric motor 6 in accordance with the steering-torque detection signal and the motor current detection signal.
In the above system for controlling the electric power steering, the motor current command-value signal is inputted in digital form to the controller CT. That is, the motor current detection signal is converted into a digital signal for addition to the motor current command-value signal.
Thus, it is necessary to convert the motor current detection signal from analog form into digital form. The AD conversion means 15 converts from an analog signal into a digital signal as follows.
FIG. 8 shows a graph illustrating the relationship between the input signal before the AD conversion and the output signal after AD conversion where the motor current detection signal is concerned. Specifically, in FIG. 8, the horizontal axis indicates the input signal of the motor current detection signal and the vertical axis indicates the output signal thereof.
The solid line A in FIG. 8 represents the output when an analog signal represented by the two-dot chain line B is inputted to the AD conversion means 15, and forms a staircase shape as illustrated in FIG. 8. The reason why the output signal forms such a staircase shape will be described next.
The AD conversion means 15 includes a conversion circuit with 10-bit resolution. Specifically, the number of divisions of a digital signal is 2 raised to the tenth power=1024 bits. The motor current is detected in the range of from xe2x88x9280A to +80A in agreement with the output range of the motor current. Hence, the amount of change in the motor current per bit results in 160A/1024 bits=0.156A/bit.
Since the amount of change in the motor current per bit is 0.156A/bit, 1 bit is shifted every time the motor current detection signal as an input signal changes by 0.156 A. When 1 bit is shifted, the motor current detection signal output sharply increases by 0.156 A. In other words, if the motor current detection signal inputted in analog form continuously changes, the motor current detection signal outputted in digital form takes on discrete values only every 0.156 A.
The points of the sharp changing of the output signal, e.g., points a, band c in FIG. 8, are hereinafter referred to as xe2x80x9cchange pointsxe2x80x9d.
The fact that the motor current detection signal after the AD conversion takes on discrete values for the amount of change as described above, may produce the following problems
When a vehicle travels straight ahead, the driver holds the steering wheel W around the middle position. At this point, the driver typically grips the steering wheel W loosely, and also, to be exact, he/she repeatedly turns the steering wheel W by extremely small degrees in a right or left direction. In other words, the driver oscillates the steering wheel win the right-left direction by extremely small degrees about the middle position.
Such turning of the steering wheel W by extremely small degrees as described above, causes the motor current detection signal to vary in amplitude between positive values and negative values about the zero point. If the above change point is within the amplitude range of the motor current detection signal, the output signal after the AD conversion repeats a sharp increase and decrease every time the motor current detection signal passes through the change point.
Further, as described above, the output signal after the AD conversion is added to the motor current command-value signal to form the motor current control signal. Hence, if the output signal after the AD conversion repeats a sharp increase and decrease, the motor current control signal also repeats a sharp increase and decrease. In addition, the motor current flowing through the electric motor 6 also repeats a sharp increase and decrease.
Such a repeating of the sharp increase and decrease in the motor current causes a problem of a pulsing motion being produced in the steering wheel W. If the pulsing motion is produced in the steering wheel W positioned around the middle position, the driver feels discomfort on his/her hands gripping the steering wheel W, because, due to not only a small motor current but also a small steering force and a small reaction force from the travelling wheels when the steering wheel W is positioned around the middle position, even if the motor current changes only slightly, the pulsing motion is readily transferred to the driver""s hands gripping the steering wheel W.
On the other hand, for preventing the pulsing motion, it is necessary to reduce the amount of change in the motor current per bit in the AD conversion. In turn, for reducing the amount of change in the motor current per bit, one idea is to use an AD conversion circuit with more than 10-bit resolution. However, an AD conversion circuit with more than 10-bit resolution is costly, and this raises the cost of the system for controlling the electric power steering. Thus, a high-cost AD conversion means cannot be used.
It is an object of the present invention to provide a system for controlling electric power steering in which, when a motor current detection signal is small, the resolution of the signal is increased for achieving control without a pulsing motion occurring.
The present invention is based on a system for controlling electric power steering which includes: an electric motor for generating an assist force; steering torque detection means for detecting steering torque; basic assist command-value determination means for determining a basic assist command-value from a steering torque detection signal; steering torque differentiation command-value determination means for determining a steering torque differentiation command-value from the steering torque detection signal; motor current detection means for detecting output of the electric motor; and output control means for controlling output of the electric motor, in which the output control means controls the output of the electric motor in response to a motor current control signal at a reference level resulting from the addition of a motor current detection signal and a motor current command-value signal consisting of the basic assist command-value and the steering torque differentiation command-value.
The feature in a first aspect of the present invention is that the system includes: signal level decision means for selecting a reference level or an amplification level in response to the steering torque detection signal or the motor current command-value signal; first adjustment means for carrying out the selection of the reference level or the amplification level for a signal level of the motor current command-value signal; second adjustment means for carrying out the selection of the reference level or the amplification level for a signal level of the motor current detection signal; analog-to-digital conversion means for converting the signal outputted from the second adjustment means into a digital signal; and third adjustment means for adjusting the motor current control signal to be at the reference level, in which each of the first adjustment means and the second adjustment means decides the signal level in response to a signal level decision signal sent from the signal level decision means, and the third adjustment means adjusts the motor current control signal to be at the reference level in response to the signal level decision signal and inputs the adjusted signal to the output control means.
According to the first aspect, when the steering wheel is around the middle position and the motor current detection signal is converted into a digital signal, the signal level is switched to allow the conversion at high resolution. For this reason, the motor current detection signal in digital form has a minimum of sharp increases or decreases as compared with the conventional example. Minimizing the sharp changes in the motor current detection signal allows the motor current control signal to be more strictly controlled as compared with the conventional example. This allows a more strict control for the motor current output in response to the motor current control signal.
As a result, when the steering wheel is held around the middle position, it is possible to prevent the pulsing motion.
The feature in a second aspect based on the first aspect is that the system further includes signal amplification means with amplification factor N provided at the output of the analog-to-digital conversion means for multiplying the motor current detection signal after the AD conversion by N times, wherein the steering torque differentiation command-value with respect to the steering torque detection signal is multiplied by N times to he defined as a steering torque differentiation command-value determination signal, and the basic assist command-value with respect to the steering torque detection signal is multiplied by N times to be defined as a basic assist command-value determination signal, and the signal resulting from the addition of the steering torque differentiation command-value determination signal and the basic assist command-value determination signal is defined as a motor current command-value signal.
According to the second aspect, the basic assist command-value determination means multiplies a basic assist command-value with respect to a steering torque detection signal by N times and outputs the result as a basic assist command-value determination signal. The steering torque differentiation command-value determination means multiplies a steering torque differentiation command-value with respect to the steering torque detection signal by N times, and outputs the result as a steering torque differentiation command-value determination signal. With this design, the motor current command-value signal resulting from the addition of the basic assist command-value determination signal and the steering torque differentiation command-value signal can be more strictly controlled. It is possible to achieve the control with a minimized pulsing motion in the range of the full output of the motor current.
The feature in a third aspect based on the first or second aspect is that the system further includes a filter provided minimally at either one of the outputs of the steering torque differentiation command-value determination means and the basic assist command-value determination means.
According to the third aspect, a filter is provided minimally at either one of the outputs of the basic assist command-value determination means and the steering torque differentiation command-value determination means. With the filter, not only the motor current command-value signal but also motor current control signal are smoothed. This achieves the control with the hardly any pulsing motion produced in the range of the full output of the motor current.
The feature in a fourth aspect based on the aforementioned aspects is that when the signal level decision means switches the signal level in response to the steering torque detection signal or the motor current command-value signal, the switching from the reference level to the amplification level and the switching from the amplification level to the reference level are performed on different steering torque detection signals or motor current command-value signals each of which the signal level is to be switched.
According to the fourth aspect, the signal level decision means has hysteresis characteristics for the signal level determination. Due to the hysteresis characteristics provided for the signal level determination, the motor-current output can be prevented from producing sharp increases or decreases at the point of switching the signal level. As a result, if the motor-current output changes in amplitude around the point of switching the signal level, the pulsing motion is not produced in the steering wheel.